Thermal dosimetry characteristics of deep regional heating of non-muscle invasive bladder cancer

Abstract

Purpose: The aim of this paper is to report thermal dosimetry characteristics of external deep regional pelvic hyperthermia combined with intravesical mitomycin C (MMC) for treating bladder cancer following transurethral resection of bladder tumour, and to use thermal data to evaluate reliability of delivering the prescribed hyperthermia dose to bladder tissue.

Materials and methods: A total of 14 patients were treated with MMC and deep regional hyperthermia (BSD-2000, Sigma Ellipse or Sigma 60). The hyperthermia objective was 42° ± 2 °C to bladder tissue for ≥40 min per treatment. Temperatures were monitored with thermistor probes and recorded values were used to calculate thermal dose and evaluate treatment. Anatomical characteristics were examined for possible correlations with heating.

Results: Combined with BSD-2000 standard treatment planning and patient feedback, real-time temperature monitoring allowed thermal steering of heat sufficient to attain the prescribed thermal dose to bladder tissue within patient tolerance in 91.6% of treatments. Mean treatment time for bladder tissue >40 °C was 61.9 ± 11.4 min and mean thermal dose was 21.3 ± 16.5 CEM43. Average thermal doses obtained in normal tissues were 1.6 ± 1.2 CEM43 for the rectum and 0.8 ± 1.3 CEM43 in superficial normal tissues. No significant correlation was seen between patient anatomical characteristics and thermal dose achieved in bladder tissue.

Conclusions: This study demonstrates that a hyperthermia prescription of 42° ± 2 °C for 40-60 min can be delivered safely to bladder tissue with external radiofrequency phased array applicators for a typical range of patient sizes. Using the available thermometry and treatment planning, the BSD-2000 hyperthermia system was shown to be an effective method of focusing heat regionally around the bladder with good patient tolerance.

Figures

Figure 1

Typical temperature plot for a single treatment session (Patient 13, Sigma Ellipse) showing temperature readings for all 8 monitored locations (bladder, rectum, rectum mapping, spine, abdomen, buttocks, inner thigh, mouth). Note the oral probe was inserted into the mouth to record core temperature only twice (at 21 and 58 minutes into treatment) and was floating in air (offscale) for the remainder of treatment. The lowest temperatures in the cycling rectal map were obtained at the most proximal portion of the rectum near the anus.

Figure 2

Axial patient CT illustrating 3 patient anatomical characteristics – anterior-posterior (AP) separation, bladder depth (to largest cross sectional diameter), and anterior fat layer thickness – used to examine possible correlations between patient anatomy and heating.

Figure 3

Average treatment time with temperatures >40-45°C (bars displayed left to right) delivered to bladder per treatment for each patient. “Overall” indicates average across all patients. Temperatures within 40-44°C fall within the hyperthermia prescription for this study. Error bars indicate standard deviation.

Figure 4

Range of thermal dose (CEM43) per patient delivered to (A) bladder, (B) rectum, and (C) superficial normal tissues. Boxes indicate quartiles, diamonds indicate mean values, and whiskers indicate maximum and minimum values. In the case of plots without boxes (i.e., patients 2, 5, and 11 in B), the first, second, and third quartiles were all equal to 0 CEM43. X-markers indicate the number of treatments for each patient, using the scale at right.

Figure 4

Range of thermal dose (CEM43) per patient delivered to (A) bladder, (B) rectum, and (C) superficial normal tissues. Boxes indicate quartiles, diamonds indicate mean values, and whiskers indicate maximum and minimum values. In the case of plots without boxes (i.e., patients 2, 5, and 11 in B), the first, second, and third quartiles were all equal to 0 CEM43. X-markers indicate the number of treatments for each patient, using the scale at right.

Figure 5

Thermometry plots illustrating the effects of thermal steering. A (Patient 14, Sigma 60) and B (Patient 13, Sigma Ellipse) demonstrate effective separation of temperatures between bladder and rectum in both applicators due to effective treatment preplanning. C (Patient 9, Sigma Ellipse) provides an example case where real-time temperature monitoring and feedback was used in the first 20 minutes of treatment to steer the focus of heating for more effective differential heating of bladder in the remainder of treatment. Note that temperatures varied 2-4°C along the length of rectum recorded in each cyclic map.

Figure 6

Rectal maps at steady state target temperature showing temperatures relative to distal position in the rectum across 10 treatments for a single patient (Patient 9, Sigma Ellipse). The tip of the mapping catheter (distal position = 0 cm) was located adjacent to the bladder target. This particular case demonstrates use of the rectal temperature mapping data from the first four treatments (A) to evaluate the patient plan and adjust the Sigma applicator position for improved focus of heating on the bladder target in subsequent treatments (B).

Figure 7

Plots of the average time >40°C, 41°C, and 42°C (with higher values suggesting greater ease of heating) with respect to the anatomical characteristics of patient AP separation, bladder depth, and anterior fat layer thickness.

Figure 7

Plots of the average time >40°C, 41°C, and 42°C (with higher values suggesting greater ease of heating) with respect to the anatomical characteristics of patient AP separation, bladder depth, and anterior fat layer thickness.